The invention relates to a method and apparatus for using reduced sized measurement devices with balloon assist catheters. In particular, the invention relates to an improved method and apparatus for controlling the inflation of a balloon assist catheter by using a pressure sensor to determine when to inflate and deflate the balloon to obtain maximum circulation effects.
Moreover, the invention relates to a method and apparatus for measuring mean blood pressure using a cardiac assist balloon catheter. In particular, this aspect of the invention relates to an improved method and apparatus for detecting mean pressure and using that measurement to reconstruct a high fidelity blood pressure waveform.
Catheter tip measurement devices are catheters that have measurement sensors located at or near their distal tips. These devices are used in a variety of applications to measure internal properties of internal tissues and fluids such as blood volume, velocity, and pressure. Catheter tip measurement devices may be introduced directly into arteries, veins, or other body organs either by themselves or through other catheters that have been previously positioned within a patient. Catheter tip measurement devices generally have electrical or fiber optic connectors at the proximal end of the catheter that communicate data from the measurement sensors to external processing devices. One type of catheter tip measurement device is a catheter tip pressure transducer, which has at least one pressure transducer located at or near the distal tip of the catheter.
The size of the catheter tip is important, and for many applications, this size is the primary limiting factor that determines whether a measurement catheter may be used in a particular application. For example, size is important and is a limiting factor in measuring pressure within small vessels, such as coronary arteries. Size is also important where a catheter tip measurement device is being introduced through the lumen of another catheter. One such application is where a small sized catheter tip pressure transducer is introduced through the lumen of an electrode or conductance catheter. A conductance catheter has electrodes disposed at the distal end of the catheter to measure the resistivity of the blood, thereby determining the heart chamber volume. These measurements can be translated into volume and impedance measurements of heart segments on a beat-by-beat basis. A catheter tip pressure transducer introduced through the lumen of a conductance catheter allows for simultaneous measurement of pressure at the tip of the conductance catheter. In this way, the conductance catheter can be used for volume measurements, and the catheter tip pressure transducer can be used for pressure measurements. The resulting pressure/volume loops are of significant diagnostic value in many types of heart disease.
Present small-size catheter devices capable of making internal pressure measurements take the form of fluid-filled devices, electrical strain gauge type transducer devices, and fiber optic devices. Fluid-filled devices may have very small construction, but such devices provide poor measurement fidelity. Similarly, fiber optic devices may have a very small sensor size, but such devices have relatively unstable and unacceptable performance. In contrast, strain gauge type transducer devices, which utilize semiconductor pressure transducers, provide high-fidelity measurements but suffer from requiring a significantly larger feature size.
In addition to catheters with pressure transducers and other measurement devices at their distal tip, guidewires exist with measurement devices at their tip. Guidewires may be inserted into body organs and used to guide the insertion of a variety of catheters into the human body. Often catheters are too large and bulky to introduce them directly into arteries, veins, or other body organs. Therefore, smaller, more flexible guidewires are introduced into the body. Then, a catheter is slipped over the guidewire for insertion. The guidewire guides the catheter into the artery, vein, or body organ.
Guidewires may also be used to exchange catheters into arteries, veins, and body organs (referred to as xe2x80x9cexchange guidewiresxe2x80x9d). When a catheter must be removed and replaced, an exchange guidewire is inserted through the lumen of the catheter. The catheter is then removed, leaving the guidewire in place. The replacement catheter may then be inserted by slipping it over the existing guidewire. When a guidewire with measurement devices at the distal tip includes external equipment associated with the measurement device, it is necessary to include a connector between the guidewire and the equipment. This connector allows the external equipment to be removed from the guidewire in order to permit the exchange of catheters.
Some guidewires include measurement devices at the distal tip. However, these guidewires either do not have very small construction because of the use of the pressure sensor at the distal tip, or they lack the measurement accuracy and stability required for most applications. For example, U.S. Pat. No. 4,941,473 illustrates a guidewire with a pressure sensor at the distal end of the guidewire. The guidewire comprises an optical fiber surrounded by a helically wound metal wire. The thickness of the helically wound metal wire and the tightness of the winding determine the torsional stiffness of the guidewire. Although this construction may allow for small feature sizes, it does not provide the measurement accuracy and stability required for most applications. In particular, this guidewire uses optical fibers to connect a pressure sensor to its associated measurement equipment. This device expands and distorts when inserted into the body due to temperature variations, causing a zero level shift in the measured pressure.
Another potential application for pressure measurement devices is with balloon assist catheters. Balloon assist catheters are flexible polyurethane bladders used to assist the heart with circulating blood through the body. FIG. 1 illustrates a human heart. In operation, a balloon assist catheter would be inserted through the femoral artery, fed through the artery, and placed approximately just below the aortic notch 5 within the aorta 4. The aorta 4 is the primary artery for delivering blood from the heart 2 to the systemic circulation system. Once in position, the balloon assist catheter is ideally inflated with helium immediately after the aortic valve 6 closes. When the balloon is inflated, the aortic diastolic pressure is increased and blood is pushed through the aorta 4 away from the heart 2. As the aortic valve opens, the balloon deflates rapidly, producing a decrease in aortic systolic pressure with a consequent decrease in resistance when the left ventricle 8 attempts to pump blood through the systemic circulation system. By inflating and deflating the balloon as described, which is referred to as counterpulsation, circulation of blood through the body may be improved. For a more complete discussion of the operation and use of intraaortic balloons, see CARDIAC CATHETERIZATION AND ANGIOGRAPHY, 3rd ed., Lea and Febiger, at pp. 493-501.
The inflation and deflation cycle lasts for approximately 225 milliseconds and timing of the cycle is critical to obtain maximum circulatory effect. If balloon inflation occurs too early, backflow of blood may occur into the heart. Likewise, if balloon inflation occurs too late, maximum circulation effects may not be obtained.
Traditionally, timing of the inflation of the balloon has been done by a human operator based on an electrocardiogram as shown in FIG. 2. The electrocardiogram (ECG) provides a graphic recording of the electrical manifestations of the heart action as obtained from the body surfaces. The ECG has three predominant wave forms, commonly known as the P wave, representing atrial depolarization; the QRS complex, representing ventricular depolarization, which is coincident with contraction of the ventricles; and the T wave, representing ventricular repolarization. Ventricular contraction occurs during the ST segment, but there is no precise electrical event coincident with aortic valve closure. Therefore, an operator trying to time balloon inflation by the ECG would have to estimate a number of milliseconds after ventricular contraction (corresponding to the ST segment) to inflate and deflate a balloon. However, this method of timing fails to provide accurate timing for balloon inflation due to human error. Additionally, because the duration of the mechanical contraction event may vary considerably from beat-to-beat, other inaccuracies are introduced.
An alternate method for timing the inflation of the balloon involves using external pressure transducers to identify the closure of the aortic valve 6. As shown in FIG. 2, the second heart sound in each cycle occurs when the ventricles relax and the valves shut. When this occurs, the aorta 4 is at a high pressure and the ventricle 8 is at a low pressure. As a result of these pressure differences, blood in the aorta 4 will rush slightly backwards and slam against the aortic valve, causing vibrations (i.e., creating pressure waves that may be detected). These momentary changes in pressure that occur when the valves shut create what is known as the dicrotic notch in the aortic pressure waveform. To detect these pressure changes, an external pressure transducer can be used to identify the dicrotic notch on the aortic pressure wave form (FIG. 2). Although the use of external pressure transducers has provided some improved timing of the balloon inflation, this method suffers from problems typically associated with external pressure transducers. These problems include imprecise timing due to fluid column linkage, time delay, and poor frequency response of the pressure transducers.
Pressure sensors have also been provided at the tip of a catheter. However, these sensors suffer from some of the same problems associated with external sensors. Additionally, these sensors directly monitor pressure levels and inherently suffer from an inability to maintain long term stability of DC pressure levels. For example, variations in temperature may result in inaccurate pressure measurements due to a zero level shift in the measured pressure. Although methods exist for maintaining long term stability, the stability may be attained only at considerable expense.
Therefore, a need has arisen for a method and apparatus for accurately timing the moment when the aortic valves close. Based on this measurement, a balloon catheter can be timely inflated to provide maximum circulation effects. Specifically, a need has arisen for more precisely timing the dicrotic notch with an aortic pressure sensor. In particular, a need has arisen for a method and apparatus for accurately measuring at the tip of a balloon catheter any pressure changes associated with the dicrotic notch.
Further, a need has arisen for a method and apparatus for accurately timing the moment when the aortic valve closes that is not affected by the long-term stability of the pressure sensor.
When timing the dicrotic notch in a balloon-assist catheter, the pulse waveform is the important information required for timing the inflation of the balloon catheter. Mean blood pressure in the heart can be evaluated independently and the pulse tracing itself used for timing purposes. However, in some instances, detection of the aortic pulse wave tracing may be insufficient. Some applications require accurate high-fidelity detection of the blood pressure waveform. For example, during balloon assist therapy, an operator may desire to alter the amount of air inflating the balloon. Alternatively, the operator may wish to alter the cycle of the therapy by inflating the balloon every other heartbeat or every third heartbeat. With each of these alterations, the operator would like to measure any corresponding changes in the central blood pressure in order to evaluate the necessary adjustments.
The use of catheter-tip pressure transducers provides excellent results for detecting the blood pressure waveform; however, as previously discussed, these types of transducers suffer from an inability to maintain long-term stability of DC pressure levels. A baseline pressure must be verified from time-to-time and a correction made to compensate for any drift.
Traditionally, a baseline pressure could be measured with a fluid-filled catheter or a balloon catheter connected to an external transducer. Although these methods are capable of transmitting a mean pressure, these methods tend to distort the pulsatory pressure waveform, which may not be suitable for systems that require high-fidelity waveform detection.
Therefore, a need has arisen for a method and apparatus for accurately timing the dicrotic notch with an aortic pressure sensor while maintaining the ability to reconstruct an accurate high-fidelity blood pressure waveform. More specifically, a need exists for a method and apparatus for accurately detecting a mean blood pressure tracing and reconstructing an accurate high-fidelity blood pressure waveform using a pressure sensor at the tip of a balloon catheter.
The present invention achieves these goals with a unique and advantageous structure for a pressure measurement device that provides the desired measurement capability. In particular, the present invention relates to a balloon catheter for use within a human body. The balloon catheter includes a support surface at the distal end of the catheter body for supporting a pressure measurement device. An inflatable balloon surrounds at least a portion of the catheter body. A pressure sensor is capacitively coupled to a detection circuit for removing DC bias and detecting frequency changes occurring at the pressure sensor.
In a specific embodiment, the pressure sensor is a semiconductor pressure sensor. In another specific embodiment, the pressure sensor is a fiber optic pressure transducer.
In another embodiment, the present invention is a system for detecting the aortic pressure in an aorta. The system includes a balloon catheter and a guidewire. The guidewire inserts through the lumen of the balloon catheter and includes a body and a support surface for supporting a measurement device. A capacitively coupled detection circuit is attached to the measurement device and removes DC bias and detects frequency changes occurring at the pressure sensor.
In yet another embodiment of the present invention, the present invention is a method for improving the circulation effects of a balloon catheter used in a heart. First, a balloon catheter is inserted into the aorta of the heart. Then, aortic pressure changes are measured near the heart. When the location of the dicrotic notch in the aortic pressure is determined by detecting the AC component of the measured aortic pressure changes, the balloon is inflated.
In yet another aspect, the present invention provides a method and apparatus for improving the circulation effects achieved by using a balloon catheter while maintaining the ability to accurately detect the mean pressure. Specifically, a system is provided for reconstructing a high-fidelity waveform of the blood pressure in the aorta of a human body using a balloon catheter. The catheter body has a proximal end and a distal end with a support surface to support a pressure measurement device. A capacitively coupled detection circuit is connected to the pressure sensor for removing DC bias and detecting frequency changes occurring at the pressure sensor. An external pressure transducer located at one end of the catheter body detects the mean pressure within the inflatable balloon. The system may also include a patient monitor for displaying the high-fidelity waveform by adding the output of the capacitively coupled detection circuit and the output of the external pressure transducer. Alternatively, the system may use an oscilloscope or a computer to add and display the components of the high-fidelity waveform.
In an embodiment, the system may be used to detect frequency changes corresponding to the dicrotic notch in the aortic pressure. The detection of the dicrotic notch may be used to accurately time when to inflate the balloon catheter.
In yet another embodiment, the invention provides a method for reconstructing a high-fidelity pressure waveform with a balloon catheter used in a heart. First, a balloon catheter having an air line is inserted into the aorta of the heart. The air line is then closed and sufficient air is introduced in order to equalize the pressure in the balloon catheter and the aorta. The AC components and the mean aortic pressure are detected separately and then added to form a high-fidelity pressure waveform. The method may be performed using a disposable pressure sensor. The high fidelity pressure waveform may be displayed on an oscilloscope, a patient monitor, or other suitable display.